The initiation of DNA synthesis is a complex multi-step process that requires the coordinated actions of a number of replication initiation factors. This process begins with the ordered assembly of a pre- replication complex (pre-RC) at replication origins during the G1 phase. Studies in Saccharomyces cerevisiae indicate that the pre-RC consists of evolutionarily conserved replication initiation factors that include the origin recognition complex, ORC, a complex of sib subunits, and the Mcm2-7 proteins, a family of six homologous proteins. Initiation of DNA synthesis is activated by the successive phosphorylations of components of the pre-RC by at least two Cdks (cyclin dependent kinases) or Cdk- like kinases, Cdc28-Clb and Cdc-7-Dbf4. Initiation of DNA synthesis at replication origins is accompanied by a transition of the pre-RC to the elongation complex. As the elongation complex migrates away from the replication origin, the pre-RC is replaced by the post-replication complex (post-RC), which appears to include the ORC but not the other known components of the pre-RC. Previous studies showed that Mcm10 physically interacts with members of the Mcm2-7 proteins. We will investigate the multiple roles of Mcm10 in the assembly and disassembly of the pre-RC and its role in the migration of elongation forms in conjunction with the Mcm2-7 proteins. Significant efforts will also be devoted to the analysis of the functional relationship between Mcm10 and other components of the pre-RC. These studies should provide information about the mechanism that restricts DNA synthesis to once per cell cycle in eukaryotes. Cancer cells are characterized by their unregulated cell divisions. This study addresses a fundamental problem in the control of DNA replication and cell division using Saccharomyces cerevisiae as the model. Through understanding of normal cellular processes that regulate DNA replication, it may be possible to elucidate the molecular basis for abnormalities or defects that lead to the disease state in cancer cells.